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Diaz-Torres A.,ECT | Moro A.M.,University of Seville
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics | Year: 2014

Recent measurements of low-energy (quasi)elastic-scattering angular distribution of halo nuclei have shown a strong suppression of the Coulomb-nuclear interference peak. Examining the components of the elastic-scattering differential cross sections for 11Be+64Zn and 6He+208Pb at energies near the Coulomb barrier, this appears to be caused by a dramatic phase-change (destructive) of the reduced Coulomb-nuclear interference term due to continuum couplings. © 2014 The Authors.

Roggero A.,University of Trento | Roggero A.,National Institute of Nuclear Physics, Italy | Mukherjee A.,ECT | Pederiva F.,University of Trento | Pederiva F.,National Institute of Nuclear Physics, Italy
Physical Review Letters | Year: 2014

We present fully nonperturbative quantum Monte Carlo calculations with nonlocal chiral effective field theory (EFT) interactions for the ground-state properties of neutron matter. The equation of state, the nucleon chemical potentials, and the momentum distribution in pure neutron matter up to one and a half times the nuclear saturation density are computed with a newly optimized chiral EFT interaction at next-to-next-to-leading order. This work opens the way to systematic order by order benchmarking of chiral EFT interactions and ab initio prediction of nuclear properties while respecting the symmetries of quantum chromodynamics. © 2014 American Physical Society.

Perez-Ramos R.,University of Jyväskylä | Perez-Ramos R.,University of Valencia | Mathieu V.,ECT
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics | Year: 2013

The collimation of energy inside medium-modified jets is investigated in the leading logarithmic approximation of QCD. The Dokshitzer-Gribov-Lipatov-Altarelli-Parisi (DGLAP) evolution equations are slightly modified by introducing splitting functions enhanced in the infrared sector. As compared to elementary collisions in the vacuum, the angular distribution of the jet energy is found to broaden in QCD media. © 2012 Elsevier B.V.

Mukherjee A.,ECT | Cristoforetti M.,ECT
Physical Review B - Condensed Matter and Materials Physics | Year: 2014

Recently, a method based on stochastic integration on the surfaces of steepest descent of the action was introduced to tackle the sign problem in quantum-field theories. We show how this method can be used in many-body theories to perform fully nonperturbative calculations of quantum corrections about mean-field solutions. We discuss an explicit algorithm for implementing our method, and present results for the repulsive Hubbard model away from half-filling at intermediate temperatures. Our results are consistent with those from other state-of-the-art calculations. © 2014 American Physical Society.

Mukherjee A.,ECT | Cristoforetti M.,ECT | Scorzato L.,ECT
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2013

We propose a new algorithm based on the Metropolis sampling method to perform Monte Carlo integration for path integrals in the recently proposed formulation of quantum field theories on the Lefschetz thimble. The algorithm is based on a mapping between the curved manifold defined by the Lefschetz thimble of the full action and the flat manifold associated with the corresponding quadratic action. We discuss an explicit method to calculate the residual phase due to the curvature of the Lefschetz thimble. Finally, we apply this new algorithm to a simple one-plaquette model where our results are in perfect agreement with the analytic integration. We also show that for this system the residual phase does not represent a sign problem. © 2013 American Physical Society.

Cristoforetti M.,ECT | Di Renzo F.,University of Parma | Scorzato L.,ECT
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2012

It is sometimes speculated that the sign problem that afflicts many quantum field theories might be reduced or even eliminated by choosing an alternative domain of integration within a complexified extension of the path integral (in the spirit of the stationary phase integration method). In this paper we start to explore this possibility somewhat systematically. A first inspection reveals the presence of many difficulties but-quite surprisingly-most of them have an interesting solution. In particular, it is possible to regularize the lattice theory on a Lefschetz thimble, where the imaginary part of the action is constant and disappears from all observables. This regularization can be justified in terms of symmetries and perturbation theory. Moreover, it is possible to design a MonteCarlo algorithm that samples the configurations in the thimble. This is done by simulating, effectively, a five-dimensional system. We describe the algorithm in detail and analyze its expected cost and stability. Unfortunately, the measure term also produces a phase which is not constant and it is currently very expensive to compute. This residual sign problem is expected to be much milder, as the dominant part of the integral is not affected, but we have still no convincing evidence of this. However, the main goal of this paper is to introduce a new approach to the sign problem, that seems to offer much room for improvements. An appealing feature of this approach is its generality. It is illustrated first in the simple case of a scalar field theory with chemical potential, and then extended to the more challenging case of QCD at finite baryonic density. © 2012 American Physical Society.

Boselli M.,ECT | Diaz-Torres A.,ECT
Journal of Physics G: Nuclear and Particle Physics | Year: 2014

This paper addresses the critical importance of an unambiguous separation of the different components of the total fusion cross-section, which is a great theoretical and experimental challenge, in order to make further progress in the field of low-energy fusion of weakly bound nuclei. Recent theoretical developments in this area are reviewed. Calculations based on a classical dynamical reaction model indicate that the contribution of sequential fusion to the complete fusion cross-section is very substantial. A toy quantum model is presented, which introduces position projection operators. These can be useful for a quantitative understanding of complete and incomplete fusion of weakly bound nuclei within a more realistic quantum model. © 2014 IOP Publishing Ltd.

Gubler P.,ECT | Weise W.,ECT | Weise W.,TU Munich
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics | Year: 2015

Moments of the ϕ meson spectral function in vacuum and in nuclear matter are analyzed, combining a model based on chiral SU(3) effective field theory (with kaonic degrees of freedom) and finite-energy QCD sum rules. For the vacuum we show that the spectral density is strongly constrained by a recent accurate measurement of the e+e-→K+K- cross section. In nuclear matter the ϕ spectrum is modified by interactions of the decay kaons with the surrounding nuclear medium, leading to a significant broadening and an asymmetric deformation of the ϕ meson peak. We demonstrate that both in vacuum and nuclear matter, the first two moments of the spectral function are compatible with finite-energy QCD sum rules. A brief discussion of the next-higher spectral moment involving strange four-quark condensates is also presented. © 2015 The Authors.

Cristoforetti M.,ECT | Scorzato L.,ECT | Renzo F.D.,University of Parma
Journal of Physics: Conference Series | Year: 2013

Recently, we have introduced a novel approach to deal with the sign problem that prevents the Monte Carlo simulations of a class of quantum field theories (QFTs). The idea is to formulate the QFT on a Lefschetz thimble. Here we review the formulation of our approach and describe the Aurora Monte Carlo algorithm that we are currently testing on a scalar field theory with a sign problem. © Published under licence by IOP Publishing Ltd.

Cristoforetti M.,E.C.T.
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2011

The spontaneous breaking of chiral symmetry and the U(1) axial anomaly in QCD can be phenomenologically understood by considering instantons as the gauge configurations mediating quark-quark interaction. The existence of an exact zero-mode solution of the Dirac equation in the field of a single instanton is the fundamental ingredient. Explicit expressions for ψ 0 are available for T 0 and μ=0, and μ 0 and T=0. In this paper we derive the solution for the most general case T 0 and μ 0. This opens the possibility of investigating the QCD dynamics associated with instantons in the full phase diagram. As a first step in this direction, we will study the dependence of the instanton density from the thermodynamic coordinates. © 2011 American Physical Society.

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